This invention discloses an advancement in the field of power control, and, in particular, in the field of transformers providing variable power for high power applications by changing tap connections on the transformer.
Apparatus to change the tap connections on a transformer under load is well known in the art and is available from several manufacturers. It is a proven, efficient, and cost-effective way to adjust voltage in high-power applications where rapid response is not required.
One usual shortcoming of available tap-changing apparatus is that only a limited range of voltage adjustment is allowed; typically xc2x110%. One reason is that there is a practical limit to the number of taps that can be provided on a transformer. With a limited number of taps, the range of adjustment can be extended only by increasing the spacing between the taps; which sacrifices resolution.
However, there are many high-power applications that need full-range control of voltage with high resolution, but do not require rapid response. Examples of such applications include electrical heating of materials in the manufacture of semiconductors and abrasives, electric refining of metals, electric plating of metals, electric melting of glass, and electrochemical production of chemicals such as chlorine. Such applications typically use electronic converters based on semiconductor switches for voltage control. These solutions have the advantage of full-range control with high resolution and rapid response; but they often have the disadvantages of harmonic currents, poor power-factor, poor efficiency, and significant waste heat.
FIG. 1 shows a prior art mechanical tap-changer. Only a single phase circuit is shown, or, more generally, one of three identical phases. The transformer secondary winding has been divided into two parts, 10a and 10b. Secondary winding 10b contains a plurality of taps. An arrangement of contacts, R, S, and T, are shown to change the tap settings of the partial winding while under load. Contacts R, S, and T are capable of opening with current flowing and of closing with voltage present. Selector switches, numbered 1-9, do not have or need this capability.
Selector switches 1-9 are arranged in two groups, one group for the odd-numbered taps 12a and one group for the even-numbered taps, 12b. If one of the odd-numbered taps is in use, contacts R and T will be closed and contact S is opened. To transfer to an adjacent even-numbered tap, contact T is first opened. Preventive auto-transformer 14 is constructed to have an impedance low enough that it can carry the load current after contact T is opened, but high enough to limit the current between taps when contacts R and S are both closed.
After contact T has opened, contact S is closed. The load current now divides between two taps, while the load voltage assumes the mean value between the two taps. Some current will circulate between the taps, but will be limited by the impedance of preventive auto-transformer 14. After contact S has closed, contact R is opened. The load current now flows entirely from the selected even-numbered tap. Preventive auto-transformer 14 carries this load current by means of its low impedance as before. Finally, after contact R has opened, contact T is closed. This shorts-out preventive auto-transformer 14 and eliminates the voltage drop due to its impedance.
Selector switches 1-9 are controlled by two separate but interlocked mechanisms, one for odd-numbered group 12a and one for even-numbered group 12b. Odd-numbered switches 12a are never changed unless contact R is opened, while even-numbered switches 12b are never changed unless contact S is opened. This ensures that no current is present on the selector-switches when they are opened, and that no voltage is present on the selector-switches when they are closed.
In FIG. 1 the selected voltage from the tapped partial winding 10b is connected only to boost or add to the voltage from the un-tapped partial winding 10a. It is also possible to connect them to buck, or subtract. FIG. 2 shows such a configuration. In FIG. 2, reversing switches A, B, C, and D have been added so that the selected voltage from tapped partial winding 10b can either be added to or subtracted from the voltage from the un-tapped partial winding 10a. This allows a smaller number of taps to achieve the same total number of selections.
FIG. 3 shows a variation on FIG. 1, in which the windings of preventive auto-transformer 14 are separated into two half-windings, C1 and C2. Contacts R, S, and T can then be moved downstream of these windings, which allows contact T to be the only one capable of opening with current flowing or closing with voltage present. In FIG. 3 only part of the tapped winding is shown, including only two of the selector switches, B1 and B2.
FIG. 3 also shows an additional improvement over FIG. 1, in that the auto-transformer is designed to permit continuous operation while supporting the voltage between two adjacent taps. This allows the control strategy to include operating modes in which two adjacent selector switches are closed simultaneously, as shown in configuration A in FIG. 3. The auto-transformer then causes the load voltage to be the average of the two tap voltages. This has the same effect as doubling the number of taps, and improves the resolution.
This invention comprises a hybrid configuration for applications that do not require rapid response. A high-power tap-changing transformer with full range of adjustment but limited resolution is combined with a low-power electronic converter of limited range but high resolution. The electronic converter provides the ability to adjust the voltage between the spaced taps of the main transformer, so that the combination exhibits high resolution. In this arrangement, the majority of the power is processed by the tap-changing transformer, where it benefits from high efficiency, high power-factor, and the absence of harmonics. Only a small fraction of the power is processed by the electronic converter, such that its associated disadvantages are proportionately diminished.
An embodiment of the invention is disclosed in which the invention is used to ensure that the mechanical switches in the tap-changer are opened only under conditions of low current and closed only under conditions of low voltage, so that contact wear due to arcing is reduced. This allows components normally found in tap-changers for the purpose of arc-reduction to be eliminated, simplifying the mechanical apparatus and recovering part of the cost of the electronic converter.
An alternate configuration is further disclosed in which the mechanical switches in the tap-changer are replaced by semiconductor switches. This configuration of the electronic converter ensures that the semiconductor switches in the tap-changer are opened only under conditions of low current and closed only under conditions of low voltage, which simplifies the associated circuits for voltage-sharing, for dV/dT suppression, and for driving the gates. While not quite as efficient as the mechanical tap-changer, this alternative still has the benefits of high efficiency, high power-factor, and low harmonics. It may be preferred at lower power levels, or when oil-filled components cannot be employed.